Method for manufacturing multi-layer printed circuit boards

ABSTRACT

Methods and apparatus for providing fine line, high density multiple layer printed circuit board packages are disclosed. In the method for fabricating multiple layer printed circuit board package, a printed circuit board is formed having a conductive circuit pattern embedded in and integral with an insulator materil substrate, such that the surface of the conductive circuit pattern is exposed along one surface of the substrate, and lays flush and coplanar with therewith. At least two of said boards are stacked with a layer of insulator material interposed between each pair of adjacent boards. The entire assembly is heat-pressed together to form a homogenous block of insulator material having conductive circuit patterns embedded and integrally molded therein.

This application is a continuation of application Ser. No. 06/778,318filed on Sept. 20, 1985, abandoned, which is a division of applicationSer. No. 06/571,139 filed on Jan. 13, 1984, U.S. Pat. No. 4,606,787which is a continuation of application Ser. No. 06/354,736 filed on Mar.4, 1982, abandoned.

BACKGROUND OF THE INVENTION

This invention relates to a method for manufacturing dense, fine lineprinted circuit boards and multiple layer printed circuit boardpackages.

There are many methods of manufacturing printed circuit boards usedextensively throughout the electronics industry. The advent of verylarge scale integrated circuits ("VLSI") has created an ever increasingdemand for higher component density per unit of printed circuit boardarea. To meet this growing demand, printed circuit boards must befabricated having extremely narrow conductor line widths and spacings.Because of the limitations inherent in the prior art methods, theycannot successfully meet the industry demands for high yield, multilayerprinted circuit boards possessing good dimensional stability andever-smaller line widths and spacings.

Although there are many methods known and used in the fabrication ofprinted circuit boards, the most widely accepted methods employ anetching technique. Typically, these methods include the steps ofcladding a base of an electrically insulating material with a conductivecopper foil, placing a photoresist material in intimate contacttherewith, developing the photoresist material to define a conductivecircuit pattern thereon, and etching away any exposed foil which is notcovered with photoresist to provide a raised conductive circuit pattern.

This prior art method creates several problems since the conductorpatterns are not flush with the surface of the circuit board, aconductor line can be easily scratched during handling, resulting in anopen circuit Also, the copper conductor may sliver and bridge acrossadjacent conductors, causing short circuits.

Furthermore, the etching step in the prior art method may also create avariety of irregularities and defects in the printed circuitry. Etchingmay result in a conductor being overetched near its base, therebyundercutting the conductor causing a nonuniform, mushroom-shapedcross-section. Also, photoresist may become trapped beneath the mushroomledges, preventing foil hidden beneath the trapped photoresist frombeing etched away. Over-etching, therefore, makes fine line stabilityand line width control extremely difficult to achieve as line andspacing widths and tolerances grow smaller. Thus, etching fabricationmethods can result in multiple conductor line defects, significantlyreducing board yields, with a consequent upsurge in rejected printedcircuitry which increases final production costs.

Board flatness and dimensional stability are important characteristicsfor insuring that printed circuitry maintains continuous conductiveinterconnection with component leads and adjacent boards. However,temperature and pressure fluctuations that occur during lamination causethe board to warp creating considerable stresses to develop in printedcircuitry mounted on equipment rails. These stresses cause conductors tobreak and/or to "swim" off the substrate fabricated by prior art methodsbecause they have poor ductility and do not lay flush with the circuitboard.

Quality and stability of multiple layer circuit board packages is alsolimited by prior art fabrication methods. To make such packages,lamination bonding layers of insulation must be sandwiched betweencircuit board layers to fill voids between the raised conductor linesand the circuit board substrate. Filling the voids requires highpressures during the lamination process which can destructively distortthe conductor lines. Also, even very high pressures cannot insure thatthe laminate will fill all of the voids. Ultimately, many voids mayremain within the finished multilayered package which become adepository of impurities. Such impurities can cause electrical shorts.Further, the lamination bonding layers and board substrates may be ofdifferent material composition since they are often supplied bydifferent manufacturers, may be made of different resins, or come fromdifferent manufacturing runs. Consequently, the finished multilayerpackage is not homogeneous. Lack of homogeneity makes it difficult toset proper drill speeds, and drill angles in the fabrication of holesthrough the multilayers. In some cases the drill speed will be too fastto cut through the copper causing it to tear, but will be the properspeed to cut through the insulation. Thus, some of the layers will havetears, and others will be smooth and some will be extremely uneven,contributing to degraded board quality while increasing unit cost.

Achieving high density in printed circuitry also requires that auniform, continuous conductive coating be placed on small diameter holesthat are drilled through multilayer packages for component leads andinterconnects. One technique widely known in the art for making holewalls conductive is electroless plating, whereby an electroless metaldeposit, usually of copper, is uniformily coated on the dielectric boardsubstrate. This technique has the disadvantage of depositing a coatingthat has poor adhesion qualities so that additional steps to insureadequate adhesion are required Another technique is the application of athin electroless coating to the hole wall, then eletroplating to furtherbuild up the conductive surface. Conventional electroplating techniques,however, cannot access holes having small diameters and large depthsdemanded by fine line, high density printed circuitry. Therefore theprior art teaches coating very small diameter holes, such as 0.0115 orless totally by electroless processing which takes a substantial amountof processing time - 24 hours or more.

SUMMARY OF THE INVENTION

The present invention includes a method of manufacturing fine line, highdensity printed circuit boards and printed circuit board packages andthe circuit boards and packages formed according to the method. A flashlayer of conductive material, preferably copper, is electrodepositedonto a rigid metal or metallized substrate that has a low coefficient ofthermal expansion. A thickness of of photosensitive resist is depositedonto the first layer by silkscreening or other methods known in the art.A mask is placed over the resist to define a conductive circuit patternon the surface of the resist. The mask is exposed to light, and theresist is developed. Channels having straight and parallel walls ofresist will be formed defining the conductive circuit patternduplicating the photomask thereby exposing the flash layer.

A second layer of conductive material is built up on the physicallyexposed portions of the flash layer of conductive material within thechannels, forming a raised conductive circuit pattern having a thicknessnot exceeding the depth of the channels. The remaining photosensitiveresist is then removed from the flash layer.

The flash layer and the second layer defining a raised conductivecircuit pattern are completely covered with a uniform layer of insulatorlaminate material. Pressure is applied to fully embed the raisedconductors in the insular material, such that the flash layer ofconductive material remains in intimate and continuous contact with theinsulator material.

The flash layer, integrated with the raised conductive circuit patternand the insulator material, is separated from the rigid substrate. Theflash conductive layer is then etched away, so that the conductivecircuit pattern embedded in the insulator material is exposed as layingflush and coplanar with the surface of the insulator material.

Printed circuit boards may be formed having embedded conductors exposedon a single side. However, a double sided board may be fabricated ifdesired, by heat presseing two such printed circuit boards together,back to back or by embedding the conductors on both sides of a singleboard.

The completed circuit boards are stacked with a layer of insulatorlaminate bonding material interposed between each printed circuit boardlayer. The multiple layers of printed circuitry and interposed insulatormaterial are heat-pressed together to form a homogeneous package ofinsulator material with conductive circuit patterns embedded therein.

To prepare the boards for components and interconnects, holes aredrilled through the homogeneous package. The holes are coated with athin layer of conductive material, preferably copper, using anelectroless coating method so as to provide a conductive substrate forelectrodepositing additional conductive material thereon. Using a highimpingement speed electrodepositing apparatus, a continuous and uniformthickness of conductive material is plated onto the walls of the holes.

A primary object of this invention is to provide a method formanufacturing fine line, high density printed circuitry whereby theconductive circuit pattern lays flush and aligned with its insulativesubstrate.

It is a further object of this invention to provide a method formanufacturing fine line, high density printed circuitry whereby theconductor lines have improved ductility characteristics.

It is a further object of this invention to provide a method formanufacturing fine line, high density printed circuitry that hasimproved dimensional stability.

It is a further object of this invention to provide a method formanufacturing fine line, high density printed circuitry whereby theconductive circuits have a uniform width along its cross-section.

It is a further object of this invention to provide a method formanufacturing fine line, high density multiple layer printe circuitboard packages of uniform insulator material which have flat, stable,warp-free, and void-free characteristics.

It is still a further object of this invention to provide a method forfabricating fine line, high density printed circuit board packageshaving small diameter through-holes of uniform and continuous conductivewall thickness.

Other objects of the invention will become more apparent upon a readingof the following description together with the accompanying drawing inwhich like reference numerals refer to like parts throughout.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a side view of a substrate with a flash layer depositedthereon.

FIG. 2 is a side view of the substrate in FIG. 1, having a layer ofphotoresist deposited thereon.

FIG. 3 is a perspective view of the assembly in FIG. 2 with a photomaskaligned thereon.

FIG. 4 is a perspective view taken along line 4--4 of FIG. 3illustrating rectangular channels defining a conductive circuit patterndeveloped in the photoresist layer after the photomask is removed.

FIG. 5 is a side view of raised conductor lines deposited within thephotoresist channels onto the flash layer taken along line 4--4 of FIG.3.

FIG. 6 is a perspective view of the raised conductive circuit pattern ofFIG. 5 after the remaining photoresist is removed.

FIG. 7 is a view taken along line 4--4 of FIG. 3 showing a insulatorlaminate layer covering the raised conductive circuit pattern and flashlayer of FIG. 6.

FIG. 8 is a view taken along line 4--4 of FIG. 3 showing the assembly ofFIG. 7 removed from the rigid substrate.

FIG. 9 is a top plan view of the printed circuit board assembly of FIG.8 with the flash layer etched away thereby exposing the conductivecircuit pattern embedded in and aligned flush with the insulatorlaminate.

FIG. 10 is a cross-section of the printed circuit board assembly takenalong lines 9--9 of FIG. 9.

FIG. 11 is a side view of a printed circuit board assembly withregistration holes drilled therethrough.

FIG. 12 is a perspective cross-sectional view illustrating multipleprinted circuit boards and insulator laminate layers interposedtherebetween stacked upon a conventional press.

FIG. 13 is a perspective view illustrating a homogenous multi-layerprinted circuit board package of the present invention.

FIG. 14 is a partial perspective view illustrating electrodeposited andelectroless plating layers built up on a interconnect or lead hole ofthe present invention.

DETAILED DESCRIPTION OF THE INVENTION

Referring first to FIG. 1, a substrate 10 is comprised of a materialsuch as stainless steel having a metallized surface for receiving aflash layer of electrodeposited material. In the preferred embodiment,the substrate is of a rigid metal or a metallized plate. However, manyother rigid material compositions having suitable characteristics may beused, such as a metallized glass material for example metallized Pyrex,having a very low coefficient of thermal expansion. The substrate musthave a low coefficient of thermal expansion to insure that when aconductor is placed thereon, it will not shift or float from its designpositions due to the thermal expansion of the substrate caused during asubsequent pressing step.

A flash of electrically conducting material 12, preferably copper, iselectroplated onto the substrate 10. The copper flash 12 serves as abase layer upon which further electroplating of conductor lines may beapplied. It also serves as a releasing material for separating theprinted circuitry from the stainless steel substrate 10 after formationof the printed circuit board is complete, as will be described in detailhereinbelow.

The flash layer is as thin as can be since heat transfer characteristicsof a very thin layer tend to rapid heat dissipation during heating,causing improved conductor line stability. Consequently, a flash layerof only 0.0001" to 0.0002" is deposited onto the substrate in thepreferred embodiment. Furthermore a very thin layer is less wasteful ofcopper. The thin electroplated coating is achieved by utilizing anelectroplating apparatus commonly known as a high impingement speedplating apparatus, such as is taught in U.S. Pat. No. 4,174,261 known asRISP, available from Economics Laboratory, Inc., Osborn Building, St.Paul, Minn., 55102. Alternatively, any conventional electroplatingapparatus may be used for applying the copper flash to the substrate.However, known conventional electroplating apparatus generally cannotplate the extremely thin coating contemplated in the present inventionwithout causing pinholes and other imperfections, and therefore is notpreferred.

A low contact pressure is desired at the interface between the flashlayer and the substrate to facilitate separating the flash layer fromthe substrate. Low contact pressure may be accomplished by usingdissimilar materials for the flash and for the substrate, such as, butnot limited to, using a copper flash with a stainless steel substrate asin the preferred embodiment. Alternatively, materials having similarsurfaces may be used if either material is coated with an impurity forreducing adhesion at their interface.

In the preferred method of the invention, the printed circuitry may befabricated on one side of the stainless steel substrate, but formultilayer applications may be fabricated on both sides of thesubstrate. This facilitates maximum production output, and allowsoptimal utilization of electroplating and other apparatus used in themethod.

Referring generally to FIGS. 2 through 5, a layer of photosensitiveresist material 14, such as Dryfilm, manufactured by Dupont, is appliedto the copper flash surface 12 of the substrate 10 using techniques wellknown in the art. The photoresist will be either positive, such that itdissolves when exposed to light, or negative, i.e., it will not dissolvewhen exposed to light. A photomask 16 defining a conductive circuitpattern 18, is placed on top of the photoresist layer 14, by techniqueswidely known in the art. The photomask 16 is aligned and brought intocontinuous contact with the surface of the photoresist 14 to insure ahigh resolution of the conductive circuit pattern on the surface of thephotoresist 14. The photomask masks the surface of the photoresist suchthat when it is exposed to light only the areas in which the conductorsare to be defined are left exposed.

After the photomask 16 is exposed to light, it is removed, and thephotoresist 14 is developed using a commercially available developersuch as Resist Stripper manufactured by Dupont. As a result, cavities20, are formed in the areas where the photoresist 14 dissolved exposingthe copper flash 12 previously covered by said photoresist 14 in thedefined conductive circuit pattern 18. The walls of cavities 20 areparallel to each other and perpendicular to the substrate 10, amountingto essentially rectangular channels running throughout the remainingundissolved photoresist 14 according to the original conductive circuitpattern 18 defined by the photomask 16.

The entire assembly is placed in a high impingement speed platingapparatus which in the preferred method is the RISP apparatusmanufactured by Economics Laboratory, Inc., St. Paul, Minn. A conductivematerial 26, such as copper, is electrodeposited onto the exposed copperflash 12 at the bottom of the rectangular channels 20, rather thanutilizing the subtractive etching method as taught in the prior art. Theelectrodeposited material 26 is accumulated within the channels to adesired thickness of about 1.2 to about 15 mils, the thickness beingselected to prevent mushrooming of the electrodeposited material ashappened in the prior art. At no time, however, should the thicknessexceed the depth of the channels. The additive electroplating stepproduces conductor lines 26 having straight and perpendicular side wallsof uniform cross-sectional width, facilitating fine line resolution, andmaking it possible to easily control line widths and densities ofextremely narrow dimension. The use of the RISP apparatus enables theconductor lines to be plated with speed and uniformity that isconsiderably better than can be achieved with conventionalelectroplating techniques. Additionally, rapid impingement speedelectroplating produces a very ductile conductor which is critical inpreventing defects and failures in very narrow cross-sectioned conductorlines.

Referring generally to FIGS. 5 through 8, the photoresist layer 14 ischemically stripped away from the copper flash surface 12 exposing theraised electroplated conductive circuit lines 26 arranged in pattern 18.A layer of insulating material 32, such as epoxy coated fiberglass, islaminated to the copper flash layer 12 completely covering the copperflash 12 and the raised conductive circuit pattern 18. Thermosettinginsulator materials such as epoxy coated fiberglass are utilized becauseof their low cost and good temperature characteristics. If epoxy coatedfiberglass is not used, alternative materials, such as polypropylene,phenolics, or Teflon material manufactured by Dupont may be used.

The insulating layer 32 is laminated over the conductive circuit pattern18 and the copper flash layer 12 by the application of heat and pressureas required for laminate material chosen, accomplished with a rigidpattern laminating press such as manufactured by Pasadena Hydraulic ofEl Monte, Calif. When epoxy coated fiberglass is used this laminationstep can be performed at a pressure of approximately 50-250 pounds persquare inch depending on the weave of the glass fabric. (Thicker glassrequires more pressure to set the epoxy into the weave) and at atemperature of approximately 425 degrees Fahrenheit. The insulatingmaterial 32 will thereby flow and completely fill all the voids betweenthe raised conductor lines 26 and will also achieve a strong bond withthe conductors. The insulator material 32 should be of uniform thicknessso that the conductor lines 26 of the circuit pattern 18 will becompletely covered by the insulator material.

The insulator material 32, in which the conductive circuit pattern 18 ismolded and embedded and which is bonded to the copper flash layer 12, ismanually separated from the surface of the substrate 10. The copperflash layer 12 is then removed from the insulator material 32 usingconventional etching techniques or a rapid impingment speed etchingapparatus, thereby exposing the conductors 26 of the circuit pattern 18embedded in the insulator material. FIGS. 9 and 10 show the resultingprinted circuit board. As illustrated in FIG. 10, the conductors 26 ofcircuit pattern 18 lays flush and coplanar with the surface 34 of theinsulator material 32, having no abutting edges or protruding surfaces.Thus, the conductive circuit pattern is totally restricted and cannotmove. This contrasts with the floating or shifting tendencies thatcommonly plague printed circuitry which have been fabricated using priorart methods, having the conductive circuit patterns raised above aninsulator material base. The embedded conductor configuration thatresults from the present inventive method provides a durable and highlystable assembly, enabling large continuous sheets of printed circuitryto be manufactured at extremely close tolerances.

The oxide created on the copper conductors does not bond well to theinsulation material. Therefore, the whole board 36 of the conductivecircuit pattern 18 is immersed in a chemical bath, such as commerciallyavailable under the trademark Macublack from McDermott of Waterbury,Conn. The chemical coating improves the adhesion qualities of thelaminate, further insuring that, if the board is stacked, the coppersurface of one board will adhere to the the laminate surface of anadjacent board. This is particularly important for boards with surfacesexposing mostly copper and thus very little laminate, such as in groundand power boards.

At this stage of the invented process, a single layer of printedcircuitry is complete. Having manufactured printed circuitry with thedesired conductor patterns, multiple layer printed circuit packages maybe fabricated. A layer of insulator material 44 is sandwiched betweeneach layer of printed circuitry 42. This insulator material 44 is of thesame composition as that used in the laminate structure of the printedcircuitry. Thus, a multiplicity of printed circuit board layers 42 arestacked atop one another, with interposed layers of insulator material44 sandwiched therebetween.

Referring generally to FIGS. 9 through 13, registration holes 38 aredrilled through each printed circuit board 42 and insulation layer 44that will be included in the multiple layer printed circuit package. Anoptically guided sighting system, such as that made by Sportonics, ofRockford, Ill. sites the target at which the hole should be made andthen drills through the targets such that there is one hole per target.The registration holes 38 provide mounting means for stacking theprinted circuit boards 42 and insulation layers 44 on mounting posts 46so that the multiple layers of printed circuitry will align securelybetween a pair of pressure plates 48 (only one illustrated).

The multiple printed circuit board layers 42, with insulator material 44sandwiched therebetween, are pressed together between the pair ofpressure plates 48 in a conventional press at a temperature of 375 to425 degrees Fahrenheit at a pressure of approximately 250 pounds persquare inch to form a multi-layer package 54. However, in the preferredembodiment, when 50 psi is used to press a single layer, the samepressure will be used throughout the process.

Prior art multiple printed circuit packages often have layers that useinsulator materials of different composition, or made in differentmanufacturing runs. Fabricating multiple-layer printed circuit boardpackages in accordance with the present method, however, enables thesandwiched insulator material layers, as well as the laminate base ofthe printed circuit board layers, to be composed of the same material.This results is a homogenous and continuous material structure when thelayers are heat-pressed together to form a multilayer printed circuitboard package 54. Also, since the printed circuit board layers of thepresent method are flush, having no protrusions or indentations,fabrication of the multilayer package is accomplished without voids orother irregularities occurring in the package structure. Further,because the surface of the each circuit board layer is flush, lowerpressure may be used to form the multilayer package. The advantage ofsuch low pressure is that warpage or distortion of the package isavoided during the pressing step.

In the prior art, internal shifting and floating that results from theuse of high pressure during the multilayer package fabrication stepplaces significant limitations on the number of circuit board layersthat could comprise a single multilayer package. In the presentinvention, the combination of developing a homogenous insulator materialin the package, along with eliminating voids through the use of flushprinted circuitry, substantially increases the number of printed circuitboard layers that can be pressed into a single package. The presentmethod has been regularly practiced on a maximum of 22 board layers, andon a maximum of 40 layers on a more limited basis. However, the methodis not thereby limited, and it is possible that packages with even agreater number of boards may be fabricated using the inventive method.

Referring to FIG. 14, interconnect and component lead holes 58 are thendrilled through the multilayer package 44 using conventional drillingmeans. The holes are generally between 0.0115 and 0.093 inches indiameter. The holes are then cleaned to remove drill smear usingcleaning means well known in the art or by the rapid impingement speedplating apparatus.

As generally illustrated in FIG. 14, a 0.000050 inch thickness of copper56 is deposited on the hole walls 58 using a conventional electrolessplating process. The copper deposit serves as a base for providingsufficient conductivity to carry substantial current for electrolysis.It should be noted that if this copper layer is too thin, it will simplyburn away due to the heat generated during electrolysis.

Once again, the holes are cleaned and rinsed to remove impurities andsurface dirt. A second thickness of copper 60 is then addedelectrolytically using the rapid impingement speed plating apparatus, tobuild up the desired conductive coating thickness along the walls of thehole 58. It is critical to this step that rapid impingement speedplating be used since conventional electroplating means cannot accessthe long and narrow diameter holes to provide a good conductive coating.Further, the use of the rapid impingement speed electroplating apparatusprovides a copper coating having improved ductility characteristics.Thus, thermal or other expansion along the vertical axis of the holewill not cause a break in the conductor surface which could interruptingcurrent flow.

The present invention is capable of fabricating line conductor widthsand spaces as narrow as 2 mils, package layers numbering 0.40 or morehaving through holes as small as 5 mils in diameter.

While certain embodiments of the present invention have been shown anddescribed above, it will be appreciated that the invention is notlimited thereto. Accordingly, it will be understood by those skilled inthe art that various changes in form and detail may be made thereinwithout departing from the spirit of the invention.

What is claimed is:
 1. A method for fabricating multiple layer printedcircuit board packages, comprising the steps of:(a) fabricating aplurality of printed cirucit board layers, wherein each layer if formedby embedding a conductive circuit pattern in an insulator material basesuch that the exposed surface of the pattern lays flush and coplanarwith the surface of the base, and wherein said conductive circuitpatterns are treated with a chemical conversion coating to provide anenhanced bond between said conductive circuit patterns and the insulatormaterial bases in which they are embedded; (b) stacking a plurality ofsaid circuit board layers on top of each other; (c) placing a layer ofinsulator material between each of said stacked printed circuit boardlayers; (d) bonding the stack of circuit boards and interposed layers ofinsulator material together to form a multiple layer printed circuitboard package.
 2. The method of claim 1, wherein fabricating eachprinted circuit board layer comprises the steps of:(a) forming auniform, thin first layer of conductive material on a polished, rigidsubstrate, wherein there is a low contact pressure between the firstlayer and the substrate; (b) depositing a thickness of photosensitiveresist onto said first layer of conductive material; (c) masking saidphotosensitive resist with a photomask to define a conductive circuitpattern on the surface of said photosensitive resist; (d) exposing saidmasked photosensitive resist to light; (e) dissolving those portions ofthe resist corresponding to the conductive circuit pattern, therebyforming three-dimensional cavities in the resist and exposing said firstlayer of conductive material according to said circuit pattern; (f)forming a second layer of conductive material upon the exposed portionsof said first layer of conductive material to form a raised conductivecircuit pattern conforming to the three-dimensional definition of thephotosensitive resist image; (g) removing previously undissolvedphotosensitive resist from said first layer of conductive material; (h)treating at least the three-dimensional top and sidewall surfaceportions of said raised conductive circuit pattern so as to provide achemical conversion coating on said surface portions, thereby enhancingthe adhesion properties of said surface portions to laminate insulatormaterials; (i) embedding said raised conductive circuit pattern on saidfirst conductive layer within a uniform thickness of laminate insulatormaterial, whereby the laminate material strongly adheres to said raisedconductive circuit pattern across the entire three-dimensional saidsurface portions thereof; (j) separating the first conductive layer withsaid attached laminate and said embedded conductive layer from the rigidsubstrate; (k) etching away said first conductive layer from saidinsulator material, whereby the conductive circuit pattern embedded inthe laminate insulator material is exposed and lays flush and coplanarwith the surface of said insulator material.
 3. The method to claim 2,wherein the step of forming a first layer of conductive material on arigid substrate comprises the steps of electrodepositing a thin coatingof conductive metal onto a rigid substrate having a metalized surfaceand a low coefficient of thermal expansion.
 4. The method according toclaim 2, wherein the step of forming a second layer of conductivematerial comprises the step of electrodepositing said material to athickness of up to 15 mils.
 5. The method according to claim 2,wherein:a rapid impingement speed electroplating apparatus is used toelectrodeposit said second layer.
 6. The method according to claim 2,wherein:the embedding step comprises laminating said insulator materialover said first and second conductive layers, and applying continuousuniform pressure and heat, thereby embedding said second conductivelayer into said insulator material.
 7. The method according to claim 1,wherein: the circuit boards and the interposed layers of insulatormaterial are bonded together by applying continuous uniform pressure andheat to form a unitary homogeneous package of insulator material withsaid conductive circuit patterns embedded therein.
 8. The methodaccording to claim 1, further comprising the steps of:(a) forming holesthrough the package; (b) coating the walls of said holes with a thinlayer of conductive material; and (c) forming a continuous conductivesurface along the walls of said coated holes.
 9. The method according toclaim 1 further comprising the steps of:(a) drilling holes for circuitcomponent electrical leads and board interconnects through said stack ofprinted circuit board layers and interposed layers of insulatormaterial; (b) using an electroless coating method for providing a thinconductive substrate layer on the walls of said holes forelectrodepositing conductive material thereupon; and (c)electrodepositing conductive material on said conductive substrate layerto form a continuous conductive surface along the walls of the coatedholes.
 10. The method according to claim 8, further comprising theadditional step executed immediately prior to coating said holes ofcleaning said holes.
 11. The method according to claim 2, wherein saidfirst layer of conductive material is electrodeposited on said substrateusing a rapid impingement speed electroplating apparatus.